Reduce number of duplicated XHR requests (w/ THREE.js', albeit limited, caching mechanisms)

[cvan]

Some are loaded only 2 times, and others are loaded 12+ times. Here are a few examples of the 191 requests:

[cvan]

We had and have several issues on file for A-Frame. @ngokevin and @dmarcos closed a ton of them by doing a lot of smart stuff to make asset management work well. There are still a ton of open issues in three.js, though there are ways to work around it (by not directly using the loaders, using in-memory objects to cache, using the Cache API [introduced by the Service Worker API], or as a last resort IndexedDB could be used).

Relevant three.js issues:

• THREE.AudioLoader load cached src  mrdoob/three.js#9323 (especially this comment)
• Replace THREE.Cache by localForage for mobile support mrdoob/three.js#8693
• Make better use of cache in loaders mrdoob/three.js#5650
• Loader Improvements mrdoob/three.js#4248
• Enhancement Proposal: Wrap OBJLoader in web worker mrdoob/three.js#9756 (good suggestion re: using Web Workers; see this WIP 👍 )